Patent literature 1: JP 2005-295658 A (corresponding to US 2005/0218860 A1)
A motor rotationally drives a rotor by using a rotation magnetic field generated by switching a current supply phase of a coil of a stator. A synchronous motor needs to switch the current supply phase of the coil, so that a rotation of the rotor is not out of synchronization with the rotation magnetic field. A rotation angle of the rotor is detected by using, for example, an encoder.
When the rotor is rotationally driven, Patent literature 1 counts the number of switching the current supply phase of the coil, and determines a rotation angle of the rotor according to the number of switching. The motor is synchronously controlled while the encoder is eliminated.
The inventor of the present application has found the following.
In Patent literature 1, since the current supply phase of the coil is switched to another in a predetermined cycle, the rotation speed of the rotor remains constant, and it may take time for a rotation position of the rotor to arrive at a target position. Operating time of the motor may become longer.